This invention relates to a process and apparatus for the production of chipboards, fiberboards, or like panels from a mixed material, the components of which have specific gravities that differ relatively greatly from one another.
The mixed material required for the production of chipboards, fiberboards, or the like, for example a mixture of chips for the cover layer and/or chips for the middle layer, combined with binders, is generally fed to a so-called bottom floor belt mixing bin, i.e. a bin, the bottom of which consists of an endless belt extending in the horizontal and drivable with a controllable speed, this bin being provided at its discharge end with an adjustably disposed scraper belt covering the entire cross section of the bin. The mixed material is then discharged from the bottom belt mixing bin in metered quantities and fed to a charging means which deposits a chip or fiber cake onto a support, such as an endless belt, which is movable in most cases. Between the bottom belt mixing bin and the feeding means, the mixed material is subjected to a weight and/or volume control to produce from the shaped cake a panel having a maximally uniform bulk density and specific gravity (German Pat. Nos. 1,088,697; 1,156,219; and DOS [German Unexamined Laid-Open Application] 1,528,234; Franz Kollmann "Holzspan-Werkstoffe" [Wood Chip Materials], Berlin-Heidelberg-New York, 1966 , p. 229).
Although the general opinion heretofore has been that the bulk density and/or specific gravity distribution within a thus-manufactured panel is sufficiently uniform with constant monitoring and continuous servicing of the entire plant, particularly the control elements, it was found unfortunately that deviations of up to 7% occur (J. Deppe and K. Ernst "Technologie der Spanplatten" Chipboard Technology, Central Wood Bulletin publishers, Stuttgart, 1964, pp. 154 and 155).
Therefore, the invention is based on the problem of reducing deviations in the bulk density and specific gravity distribution in chipboards, fiberboards, or the like to a practically negligible minimum.
The invention starts with the recognition that the mixed material to be metered is segregated not only during its transport to the bottom belt mixing bin but also during the intermediate storage in such a bin, especially if the specific gravities of the individual components of the material to be mixed deviate relatively greatly from one another. This is the case, inter alia (e.g. components of pine and beech) particularly in case of mixed materials made up of wood chips, cement, and water (U.S. Pat. No. 3,271,492) and/or mixed materials made from bagasse, hemp, flex, or the like with water and cement. If material to be mixed, which in most cases has already separated during transport, is introduced into a bottom belt mixing bin, the components of this material moistened with water as well as those saturated with water sink down primarily to the lower zone of the bottom belt mixing bin during the intermediate storage, which is undesirable.
In order to solve the above-described problem, several methods are contemplated by the present invention. Thus, it is contemplated by the present invention, especially if a strong segregation is to be expected during transportation, to feed the mixture components, for example glue-covered chips on the one hand and cement on the other, of fibers on the one hand and cement or the like on the other, separately from each other before the metered feeding to the charging station, and to deposit these substances in the bottom belt mixing bin in superimposed, relatively thin layers, the mixing step being conducted during such feeding and also after such feeding has been accomplished. The question when the required water is fed to the mixed material is of secondary significance in this connection; therefore, it is possible to combine the material with water, for example, during an intermediate storage as well as during the feeding to a charging means.
However, it is further contemplated according to another embodiment of the invention to feed the mixture components to a bottom belt mixing bin together, and then to deposit this mixture material in obliquely oriented layers in the bottom belt mixing bin, in superimposed strata, so that the angle of each layer is smaller with respect to the horizontal than the natural charging angle (angle of repose) of the individual mixture components. Here again, the components are being mixed together during the charging step and after this charging step is finished.
When speaking of relatively thin layers of the individual components and/or of relatively thin layers of the mixture material in describing the present invention, this means thickness of the individual layers of a few centimeters. If, in such a case, the thickness of the strewed chip or fiber cake is 60 mm., and this cake is then compacted, for example, to 20 mm. in a press, then differences in the bulk density and specific gravity distribution are, with a thus-produced panel, approximately 1%, so that a quite considerable advance in the art has been attained.
These two above-noted processes and devices for executing these processes will be explained hereinbelow with reference to the examples schematically illustrated in the drawings.